The present invention relates to a supercharging system for an automotive engine, and more particularly to a combined supercharging system having a compressor driven by an exhaust-gas turbine and a compressor driven by the engine itself.
Generally, the supercharging system for motor vehicles is employed with a compressor driven by an exhaust-gas turbine (hereinafter called a turbocharger). The turbocharger is effective in supercharging in a high engine speed range, but does not act to provide a sufficient supercharging pressure in a low engine speed range.
To the contrary, a compressor driven by an engine (hereinafter called a supercharger) is effective in a low engine speed range. Accordingly, a combined supercharging system provided with a turbocharger and a supercharger has been proposed in order to improve supercharging efficiency in a wide engine speed range.
Japanese Patent Laid Open No. 58-222919 discloses a combined supercharging system in which a turbocharger and a supercharger can be selectively combined either in parallel or series. However, the system is complicated in construction and, moreover, does not have practical advantages. In addition, since the system is not controlled in accordance with load on the engine, supercharging pressure is not properly controlled at light load and heavy load in a low engine speed range. Further, the condition for changing between a single supercharging range and a combined supercharging range cannot be controlled in accordance with engine operating conditions. Accordingly, both of the operational ranges are subject to deviate from desired ranges.